Why every prompt ends with "Done means"
The agent doesn't know when it's finished unless you tell it what finished looks like. The last sentence of a prompt is the one that does the most work.
Here's a prompt that looks complete and isn't:
Make a counter. Tapping the screen adds one, and there's a button that subtracts one.
Run that cold and you'll get a counter. You'll also get a subtract button that happily takes you to −1, −2, −3. Nothing in the prompt said it shouldn't. The agent did exactly what you asked. That's the trap: it did what you said, not what you meant.
The test is the spec
Now the same prompt, with the sentence that every prompt in the course ends with:
Make a counter. Tapping the screen adds one, and there's a button that subtracts one.
Done means: I tap ten times and see 10, press subtract twice and see 8, and pressing subtract at zero leaves it at zero.
Three checks. The third one is the whole reason the sentence exists. You didn't describe a rule ("never go below zero"), you described what you'll do with your thumb and what you expect to see. Agents are much better at building toward a test they can picture than a rule they have to interpret.
Describe the test, not the technology. The agent picks the technology; you pick what "working" means.
Why it goes last
Order matters less than you'd think for most of a prompt, but "Done means" goes at the end for a human reason: it's the thing you read back before pressing send. If you can't picture yourself doing each step on the phone, the prompt isn't ready.
After every prompt passes its checks, students tell the agent to save a checkpoint. A prompt that goes sideways is one step back, never a disaster.
The two clauses that came from a cold run
Day 1's first prompt has two clauses that weren't there originally: the subtract button "doesn't buzz when the count is already zero", and the number "must still fit on screen when someone has turned their phone's text size all the way up". Both were added after running the prompt in a fresh session and watching what came back. Every "Done means" in the course earned its lines that way.